This invention relates generally to the fields of semiconductors and microelectromechanical devices and processing techniques therefor, and particularly to the methods used in formation of metal microminiature structures. More specifically, the invention relates to the fabrication of components of micron or submicron dimensions using conductive polymer compositions containing metallic particles and lithographically or otherwise patterned masks. The invention pertains to miniaturization and xe2x80x9cnanotechnology,xe2x80x9d and has utility in many fields, including microelectromechanical system fabrication and semiconductor processing.
Microelectromechanical systems (commonly referred to as xe2x80x9cMEMSxe2x80x9d) are useful in a wide variety of fields and include, for example, micro-sensors, micro-actuators, micro-instruments, micro-optics, and the like. Many MEMS fabrication processes are known, and tend to fall into the two categories of surface micro-machining and bulk-micromachining. The latter technique involves formation of microstructures by etching directly into a bulk material, typically using wet chemical etching or reactive ion etching (xe2x80x9cRIExe2x80x9d). Surface micro-machining involves fabrication of MEMS from films deposited on the surface of a substrate, e.g., from thin layers of polysilicon deposited on a sacrificial layer of silicon dioxide present on a single crystal silicon substrate (this technique is commonly referred to as the xe2x80x9cthin film polysilicon processxe2x80x9d).
An exemplary surface micro-machining process is known as xe2x80x9cLIGA.xe2x80x9d See, for example, Becker et al. (1986), xe2x80x9cFabrication of Microstructures with High Aspect Ratios and Great Structural Heights by Synchrotron Radiation Lithography Galvanoforming, and Plastic Moulding (LIGA Process),xe2x80x9d Microelectronic Engineering 4(1):35-36; Ehrfeld et al. (1988), xe2x80x9c1988 LIGA Process: Sensor Construction Techniques via x-Ray Lithography,xe2x80x9d Tech. Digest from IEEE Solid-State Sensor and Actuator Workshop, Hilton Head, S. C.; Guckel et al. (1991) J. Micromech. Microeng. 1: 135-138. A related process is termed xe2x80x9cSLIGA,xe2x80x9d and refers to a LIGA process involving sacrificial layers. LIGA is the German acronym for X-ray lithography (xe2x80x9clithographiexe2x80x9d), electrodeposition (xe2x80x9cgalvanoformungxe2x80x9d) and molding (xe2x80x9cabformtechnikxe2x80x9d), and was developed in the mid-1970""s. LIGA involves deposition of a relatively thick layer of an X-ray resist on a substrate, e.g., metallized silicon, followed by exposure to high-energy X-ray radiation through an X-ray mask, and removal of the irradiated resist portions using a chemical developer. The mold so provided can be used to prepare structures having horizontal dimensionsxe2x80x94i.e., diametersxe2x80x94on the order of microns. The technique is now used to prepare metallic microcomponents by electroplating in the recesses (i.e., the developed regions) of the LIGA mold. See, for example, U.S. Pat. Nos. 5,190,637 to Guckel et al. and 5,576,147 to Guckel et al.
Unfortunately, one of the serious disadvantages of LIGA is that, unlike silicon MEMS, there is currently no way to easily and economically build multilevel LIGA devices, particularly cantilevered multilevel LIGA devices. Standard LIGA processes can only fabricate microparts that are essentially extrusions of 2-D designs. In other words, standard LIGA parts are currently prismatic. In order to microfabricate a cantilevered part, prismatic LIGA components are microfabricated separately and glued or diffusion bonded together in the proper orientation. Such post fabrication assembly raises the cost of the device considerably and results in cantilevered parts having discontinuities between the glued or diffusion bonded layers. Previous attempts at manufacturing cantilevered multi-layered LIGA microstructures, such as those discussed in U.S. Pat. No. 5,378,583 to Guckel et al., have been significantly hampered by the high interfacial stresses between the exposed and unexposed areas of photoresist that leads to extensive crack propagation when the exposed areas are developed.
U.S. Pat. No. 5,190,637 to Guckel et al. discloses a method of producing a cantilevered multi-layer microstructures by utilizing a sacrificial metal layer to surround each layer of microstructure once it has been formed. However, the formation of microstructures using this method requires a separate time-consuming electroplating step to be performed at each level, and a difficult final etching step to be performed in order to remove the sacrificial metal, thereby increasing the complexity and expense of the metal microstructure fabrication process. There is, therefore, a need in the art for a fast and efficient method of manufacturing continuous cantilevered multilevel LIGA microstructures that avoids the problems associated with interfacial stresses and time-consuming metal etching techniques.
Accordingly, the invention is directed to the aforementioned need in the art and provides a method that reduces the occurrence of stress-induced cracking when making continuous multilevel cantilevered LIGA microstructures of micron or submicron dimensions and eliminates the need for time-consuming metal etching processes.
It is another object of the invention to provide such a method that involves forming a first microstructure on the plating base, covering the first microstructure with a conductive polymer, sealing the conductive polymer with a metal layer, and forming a second microstructure on the first microstructure.
It is still a further object of the invention to provide novel microcomponents fabricated using the methodology disclosed and claimed herein.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention.
In one embodiment, then, the invention pertains to a method of forming multi-layer microstructures. In the method a first layer microstructure is established by electroplating a metal into the recesses of an exposed and developed first polymer layer and planarizing the surface of the electroplated metal to form a substantially flat and uniform surface extending across the first layer microstructure and the remaining first polymer layer. The remaining first polymer layer is then removed in its entirety. A conductive polymer layer is deposited over the first layer microstructure. The conducting polymer mitigates stress induced cracking and serves as a plating base for any cantilevered portions of the second level microstructure. The exposed surface of the conducting polymer layer is machined down to expose the surface of the first layer microstructure, forming a substantially flat, uniform surface extending across the first layer microstructure and conducting polymer layer. Machining also allows the thickness of the first layer of metal to be closely controlled.
After the first layer microstructure and conducting polymer layer have been machined down to the desired height, a metal sealing layer is deposited onto the surface of the first layer microstructure and conducting polymer layer. This metal sealing layer prevents the chemical developer used in forming the subsequent layers of microstructure from attacking the conductive polymer layer. Additional layers of microstructure are then formed using the same procedure outlined above. i.e., a second polymer layer is deposited on the metal sealing layer, areas of the second polymer layer are exposed and developed. Prior to electroplating, the areas of metal sealing layer that have been uncovered by the development of the exposed areas of the second polymer layer are also removed. The removal of areas of the metal sealing layer allows the electroplated microstructures of the second layer to form directly onto the surface of the first layer microstructure or, when the microstructure is cantilevered, onto the surface of the conductive polymer.
Using the above method, a multi-layered microstructure may be quickly and economically formed. If so desired, additional layers of microstructure may be added by covering the previous layer of microstructure with a conductive polymer layer and a metal sealing layer. The utilization of the conductive polymer layer and metal sealing layer to completely cover each layer of microstructure facilitates both the machining of newly formed microstructures and the development of cantilevered parts. The conductive polymer layer or layers may be removed from around the final microstructure by any suitable chemical and/or thermal method, i.e., dissolution in acetone or other organic solvent, heating, burning, etc.